How to come up with Portsmouth Handicap ratings

How would I go about it? I build and design a lot of custom boats that have no specific class, and I am asked for Portsmouth ratings for local regattas.

According to their website:

Portsmouth Numbers are defined as the length of time boats would take to sail a common but unspecified distance. The formula is easy to use: CT (Corrected Time) = ET (Elapsed Time) X 100 / HC (Handicap) - See more at: http://www.ussailing.org/racing/offshore-big-boats/portsmouth-yardstick/#sthash.1ObUdNk7.dpuf

So How would I go about finding my own number? Have my boat sail a standard course along with a known class vessel, and compare the two numbers, solving for the handicap of my boat based on the times of the two boats?

 

Like PHRF, Portsmouth rating are gotten from performance of boats in races. For new boats, a rating is assigned based on, you guessed it, an educated guess based on similar boats and then modified by actual performance. For a single boat it definitely cannot be very accurate. Some boats are granted a Yardstick number because their performance is so well known. Others are less accurate but are much better than you might get with a single new boat.

The formula you gave is how a particular boats finish is determined based by elapsed time relative to the elapsed time of the scratch boat in that race. The formula is not a determinant of that rating number other than a data point to be used with all the other data points in determining the Portsmouth number.

 

That is pretty much how the current numbers are arrived at. Many Clubs submit annual returns from their Handicap racing, so some sort of meaningful value can be derived. In theory, different waters and wind conditions effects are smoothed out. In reality there are some conditions which massively favour one type over another.

Best thing you can do is try out against something similar (size/type) with a number. Then over a bit of time and certainly different wind strengths asign a provisional number. Note even Primary Yardstick numbers are moved at times and some Classes are allocated these Provisional Numbers until enough data is accrued to find a meaningful value.

 

I'm not sure what you're trying to get done, but the best thing you can do is simply enter a regatta and have the sailing club sponsoring the thing give you a rate. It'll probably suck, but they'll modify it as you complete a few outings. If the boat you're getting rated is of a conventional set of shapes, the rate will be fairly reasonable, but if the boat has some oddness to it, you'll get hammered initially, until they can figure out what she can really do. Simply put, if you have a planning dinghy of conventional shapes and proportions, your rate will be pretty close. If you have a narrow gaff cat with a barndoor rudder and high buttock angles, they'll never be able to give you a reasonable rate, until they see how well it actual can perform against other displacement classes.

 

I have a similar problem so what I did was to download the excel spreadsheet on this page (in the first large section of text) http://www.capedory.org/phrf.html

In the formula is a number that is meant to be adjusted for when you do downwind /upwind or round the cans racing.

I built up a spreadsheet with several similar style boats of known handicap and entered their details into the above formula. Adjusting the fiddle factor until it matched approximately the figures for the Known PY of ALL boats.
Then I put in my own boats figures to see what I got.

I had the added complication of using UK PY and/or Norfolk UK Handicap, but it seems to work.
When my boat is refurbished and launched, I'll take the print out and bend the race handicappers ear with it, he tends to be a bit undergenerous normally!

I've also used the formula to see what happens if I modify the LWL, Sail area or displacement.

 

I've had this problem with two boats so far, a 8' mini keelboat, that will probably be about as fast as an optimist, and a 16' tri, that is somewhere between a Hobie Adventure island and a Weta.

Seems like running the course several times with another boat would be a decent way of figuring out a general range. Of course, that also depends on the crews of both boats, so it might be good to compare against say a laser and an optimist in the case of my 8' keelboat.

 

Update on this. After sailing around in my 8' Mini Keelboat, I have come to the conclusion it is very close to a walker bay 10 in speed(I had one chasing me the whole time). Sometimes it could keep up just fine, sometimes I would pull away. So for now, I guess I could tell the committee it has a DP-N of 120 just to be on the safe side, as the walker bay has a DP-N of 123.